Data communication is becoming increasingly important in today's society. For example, the popularity of the Internet has expanded demand for high quality, high speed access to a variety of services available on the Internet. Although infrastructure enabling such data communication allows transmission of data at high speeds over large distances, a barrier remains to providing high quality, high speed data communications. Communicating data between a home user or a business and a central office of a service provider requires either utilizing existing communications lines, such a phone lines, between the service provider and the home for a business user, or adding additional infrastructure, usually at significant expense.
Digital subscriber line (DSL) technology addresses the problem of connecting the central office of a service provider to a home user or business by utilizing a frequency spectrum available on existing telephone lines for data that is not used for voice traffic. The frequencies at which data are transmitted and received according to DSL technology are higher than the frequency spectrum used for voice traffic on a standard telephone line. Asynchronous digital subscriber line (ADSL) technology is a particular form of DSL. With ADSL, a different data rate is used for upstream transmission of data than is used for downstream reception of data.
DSL technology uses one of a number of technologies to encode and decode data on a single carrier frequency. These technologies include (1) a quadrature amplitude modulation (QAM), (2) carrierless amplitude and phase (CAP) modulation, and (3) discrete multi-tone (DMT) modulation. Quadrature amplitude modulation is a modulation technique using variations in signal amplitude. This modulation technique encodes data as discrete phase plus amplitude changes of a carrier tone. Carrierless amplitude and phase modulation is a modulation technique that allows multiple bits of information to be represented by a single frequency cycle. Discrete multi-tone modulation is a modulation technique that separates the frequency range in which data is transmitted and received into a plurality of frequency bands or channels, using a Fast Fourier Transform.
A problem that arises with the use of DSL technology is as distance between the central office and a user increases, the available bandwidth, and therefore, transmission rates decrease. Two reasons for this are (1) attenuation along the telephone wire, and (2) cross talk and other noise sources. Generally, the effective reach of a DSL signal over existing telephone lines is approximately three miles from the central office. Thus, a user of a DSL line must be located within three miles of the associated central office. This hinders the widespread deployment of DSL technology.